Human intelligence collection, or espionage, is what the term "intelligence" is most likely to bring to mind. Typically it involves the identifying and recruiting of a foreign official who, by virtue of a position of trust in his government, has access to important information and who is willing, for some reason, to pass it to officers of one's intelligence service. In some cases the person providing the information may not be a government official but a private individual who has the opportunity to observe (or hear about) something of interest. The person to be recruited might also be someone who is privy to important information by virtue of a close relationship with someone of intelligence interest.
Ordinarily, individuals in two different roles are involved: an intelligence officer who is an employee of an intelligence agency, and a source, who provides the officer with information. The intelligence officer, or "handler", maintains communication with the source, passes on instructions from the intelligence service’s headquarters, provides necessary resources, and, in general, seeks to ensure that the flow of information continues.